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[en] In the south of the Sierra de San Luis is recognized a basement composed by La Escalerilla and Los Puquios granites, accompanied by schist, mylonite s and mafic-ultramafic rocks. The schists, La Escalerilla granite and the mafic -ultramafic rocks are affected by a regional metamorphism in anphibolites facies, the two first present a N-S foliation di ping to east. The mylonite s correspond to a second event of deformation, distributed in thin belts of NNE-SSO direction and inclination to east, developed in green schist to anphibolites facies. Los Puquios granite represents the more young of the basement rocks and the intrusion was associated with a shear zone. Los Puquios granite forms dikes and small plutons cutting La Escalerilla granite and micas chits in high cortical levels
[en] The Sm–Nd isotope ages of apoamphibolite eclogites and eclogite-like garnet-clinopyroxene crystalline schists collected in the Chupa and Engozero segments of the Belomorian mobile belt are estimated. Eclogites and eclogite-like garnet–clinopyroxene rocks demonstrated nearly the same isotope age (2119 ± 170 and 2191 ± 39 Ma, respectively) on the same pair of minerals (garnet–clinopyroxene). The age obtained (about 2.1 Ga) is the oldest indicator of the onset of Paleoproterozoic metamorphism in the Belomorian mobile belt.
[en] The Arunachal Himalayas consist of litho-units ranging in age from Palaeoproterozoic to Quaternary with the oldest sequence of Palaeoproterozoic consisting of Se La Group and Bomdila Group. The rocks of Bomdila Group are distributed as a NNE-SSW to NE-SW trending belt in the Lesser Himalayas. Investigations for U were initiated in the study area in 1976 by AMD that has resulted in the discovery of numerous U occurrences in different lithounits in Sie Rimi- Noko Nala and adjoining areas of Tai, Kardo, Badak and Maro. Basal part of Bomdila Group, the Khetabari Formation with ferruginous, calcareous metasediments, graphitic/carbonaceous phyllite, minor carbonate and para-amphibolite, intimately associated with the Ziro gneisses has been established as a main geological horizon with Fe and U potentiality in the area. The rocks of Khetabari Formation have undergone amphibolites facies grade of metamorphism. The paper describes REE geochemistry of the uraniferous magnetite-calcite schist belonging to the above formation. (author)
[en] The Riás Schists crop out in the so-called Iberian Variscan parautochthon, surrounding the Malpica-Tui Complex (NW Iberian Massif), as part of the westernmost internal areas of the European Variscan belt. Three Variscan metamorphic events have been identified in the Riás Schists (M1, M2 and post-M2). M1 comprise unoriented microinclusions that have only been identified in garnet porphyroblast cores and inside staurolite crystals. M2, comprises the matrix foliation (S2) and the assemblage garnetRIM + staurolite + muscovite + biotite + chlorite + rutile/ilmenite + magnetite + quartz and the development of syntectonic andalusite. Finally, postM2 (post-S2) includes andalusite and plagioclase together with secondary muscovite, biotite and chlorite and accessory tourmaline, Fe/Ti oxides, apatite and carbonate. Results of multi-equilibrium thermobarometry (pressure-temperature pseudosections), for M2 indicates a medium-pressure Barrovian metamorphic event (0.5-0.6 GPa; 580-570 o C; ca. 20 km deep), followed by moderate cooling during decompression, together with the extensional collapse of the Variscan orogenic pile (post-M2; 0.3 GPa; 540 o C; ca. 10 km deep). Inferred P T conditions in the Riás Schists, and their spatial relationship with the overlying allochthonous high-pressure pelitic schists (i.e. Ceán Schists), suggest that both lithologies were part of the same continental margin during the beginning of the Variscan orogeny. However, they experienced very different ectonothermal evolutions due to their putative locations in the orogenic wedge.
[es]Los Esquistos de Riás afloran en el parautóctono del orógeno Varisco de Iberia, en el entorno del Complejo de Malpica-Tui (NO del Macizo Ibérico), en el sector interno más occidental de la Cadena Varisca Europea. Se han identificado tres eventos metamórficos en los Esquistos de Riás (M1, M2 y post-M2). M1 incluye microinclusiones sin orientación preferente que sólo se han identificado en los núcleos de los porfiroblastos de granate y en los cristales de estaurolita. M2 comprende la foliación principal de la matriz (S2) y está constituido por la paragénesis granateBORDE + estaurolita + moscovita + biotita + clorita + rutilo/ilmenita + magnetita + cuarzo y el desarrollo de porfiroblastos sintectónicos de andalucita. El evento post-M2 (post-S2) incluye andalucita y plagioclasa, además de moscovita, biotita y clorita secundarias y cantidades accesorias de turmalina, óxidos de Fe-Ti y carbonato. Los resultados de las técnicas de termobarometría multiequilibrio (pseudosecciones presión-temperatura), caracterizan M2 como un evento metamórfico Barroviense de media presión (0.5-0.6 GPa; 580-570 o C; ca. 20 de profundidad) seguido de una exhumación con enfriamiento moderado, coetánea con el colapso extensional de la pila orogénic a Varisca (post-M2; 0.3 GPa; 540 o C; ca. 10 km de profundidad). Las estimaciones realizadas en los Esquistos de Riás, y su relación espacial con los esquistos pelíticos de alta presión del alóctono (i.e. Esquistos de Ceán), sugieren que ambas litologías formaron parte del mismo margen continental durante el comienzo de la orogenia Varisca, pero experimentaron evoluciones tectonotermales muy distintas por su posición original en la cuña orogénica.
[en] The fieldwork is based on the data of strike (S0) and schistosity (S1) of cores that could not penetrate the geological structure model and result of observation on some cores has shown that U mineralization veins are not always parallel to S1. The problems were encountered in core drill data to improve the estimation of U resources from indication category to measured category. The purpose of the evaluation is to establish the advisability of geological structure model and U mineralization model which was applied by this time. The research used remapping of geological structure with surface method in the scale of 1:1000. The result of remapping shows the difference of the dipping between new geological structure model and the old model. The dipping of the new model is to South East until vertical and the old model is to North West until vertical and to South East until vertical. Despite the difference between both of them, the substantive of folding system is identical so that the new and old models can be applied in drilling in West Lemajung sector. U mineralization model of remapping result consists of 3 types : type 1 U mineralization lens form with West-East direction and vertical dipping which is associated with tourmaline, type 2 U mineralization filling in the open fractures with West-East direction and 70o to North dipping and parallel with S1, and type 3 U mineralization fill in opening fractures with N 110o - 130o E the direction and 60o to North East until subvertical dipping while the old model is only one type. It is U mineralization filling in the open fractures with West-East the direction and 70o to North the dipping and parallel with S1. Because of this significant difference, data collection of drill core must follow the new mineralization model. (author)
[en] Two fairly recent and independent events have widened the scope of search for uranium mineralisation in India. In the first place, discovery of unconformity-related deposits at the northern flank of the Cuddapah basin in southern India has renewed interest on such surfaces across the entire range of 'Purana ' basins. Secondly, the discovery of substantial uranium mineralisation in the Mesozoic sandstones at Domiasiat in Meghalaya establishes that the pre-Cambrian basement there, is potentially fertile in terms of uranium content. Taken together, these factors encourage an examination of the Shillong Group of rocks in Meghalaya. The unconformity at the base of this Group resembles the well-known one from the base of 'Purana' basins throughout peninsular India. From Sumer near Barapani towards southwest, the unconformity itself is marked by an oligomictic conglomerate while to the northeast at Umsning, a ferruginous breccia-like rock has formed. Interestingly, without any major shift across the Kopili valley, the (author)reduction in the transitithe Mikir Hills as well. Most of the rocks above the unconformity are arenaceous, in which primary sedimentary structures are retained even though the matrix has recrystallized. Semi-pelitic facies appears in the southwest from near Mawphlang; in this facies, growth of small aggregates of granular potash feldspar is noticed as well as specks of sulphide (pyrite). Increase in grade of metamorphism is noted on the plateau around the porphyritic granite exposed near Weiloi, at the southwest extremity of what has been called the Barapani-Tyrssad shear zone; in the gorge sections below Sohrarim, conglomerates with staurolite and garnet in the recrystallized interbeds crop out; further south, in the deep gorges west of Cherrapunjee-Tyrnai (Mahakalika falls) staurolite- garnet- andalusite schists close to the contact with the Sylhet Traps are found. These collectively demonstrate the influence of granite plutonism and perhaps also stratigraphic depth in controlling the thermal gradient. The type of rock in the basement immediately below the unconformity changes from place to place -from schistose near Sumer to gnessic in the southwest and almost granitoid towards Umsning. Exploration strategies in this belt need to take into account the large number of radioactive anomalies identified by the A.M.D., identification of pitchblende from the Tyrssad phyllites, the fairly steep dip of the unconformity plane below the Shillong Group, the near vertical dips along the Tyrssad-Barapani zone and the geochemically attractive nature of the intrusive plutons. Rugged terrain, high rainfall and fairly thick weathering carapace particularly below 1000 m level are limiting factors. (author)
[en] Several Proterozoic basement units crop out in the Sonora State of NW Mexico, and the same can be correlated with crustal provinces of southern Laurentia in the neighboring southwestern USA. Zircon U–Pb and Hf isotopic determinations in more than 300 grains separated from igneous and metaigneous rocks from these units indicate that the crystalline basement in Sonora is made up of different components, which are from west to east: (1) The Caborca–Mojave province to the west, characterized by the so-called Bámori Complex, have U–Pb ages between 1696 and 1772 Ma, with moderately juvenile to slightly evolved εHf values, yielding TDM ages of ca. 2.1–2.4 Ga; (2) in the intermediate area, east of Hermosillo, the Palofierral and La Ramada orthogneiss units yield an age of 1640 and 1703 Ma, respectively, both having juvenile εHf with the Palofierral overlapping the depleted mantle curve at ca. 1.65 Ga; and (3) in the northeastern Sonora, samples from the southern extension of the Mazatzal province, represented by the Pinal Schist, yielded ages between 1674 and 1694 Ma, with moderately juvenile to juvenile εHf values and a TDM age of ca. 1.9 Ga. In addition, a suite of post-tectonic granites was also studied in Caborca (San Luis granite) as well as in northeastern Sonora (Cananea granite), both yielding ages of ca. 1.44 Ga with moderately juvenile εHf values ranging from −1 to +8 and TDM dates of ca. 1.8–1.9 Ga and 1.6–1.7 Ga, respectively. These two isotopically contrasting provinces may imply the existence of a Proterozoic paleo-suture. However, if the Palofierral gneiss, of which the Hf signature straddles the depleted mantle array, is taken as the source for the 1.44 Ga Cananea granite, then the location of such a suture zone should lay farther south than the proposed trace of the Mojave–Sonora megashear.
[en] The heterogeneous assemblage of zircons was identified in metamorphic schists of the Karabash site of the West Siberian Plate. Based on the U–Pb (SHRIMP II) isotope data, zircon varies in age from the Archean–Paleoproterozoic boundary to the early Permian. The Precambrian age data obtained for the study area reflect the erosion of the Precambrian complexes in the framework of West Siberian Plate (with subsequent redeposition of the ancient detrital material within the West Siberian megabasin). Our results make it possible to clarify the history of the formation and evolution of metamorphic rocks in the basement of the West Siberian Plate.
[en] The Litchfield province is herein defined as the westernmost outcropping unit of the Pine Creek Geosyncline extending from the Giant's Reef Fault at about longitude 130050'E westerly to the edge of the Bonaparte Gulf basin. The northern half of the province has been remapped using company drillhole data and a Bureau of Mineral Resources air magnetic-radiometric survey. Five areas have been found to be entirely granitoid, garnetiferous and gneissic in part, with composition in the range adamellite to granodiorite. A single Rb/Sr age of about 1800 m.y. has been published. The granitoid margins are migmatised and transitional into large areas of metasediments, varying in metamorphic grade from greenschist (fine grained chloritic graphitic schist) to upper amphibolite/granulite grade (course grained quartz - feldspar gneiss). The regional strike is northerly to northwesterly, and these metasediments are thought to merge into the Burrell Creek Formation beyond the border of the province. Two periods of metamorphism affected the Litchfield province metasediments, but only one phase can be detected in the granitoids, thus these are assumed to be younger, probably formed during the 1800 m.y. orogenesis known elsewhere in the Geosyncline. Minor areas of unmetamorphosed gabbro, dolerite and quartz-diorite occur, probably of Middle Proterozoic age. Cover rocks include quartz sandstones of the Middle Proterozoic Moyle River Formation and Depot Creek Sandstone, and Cambro-Ordovician sandstones of the Daly River Basin. The formation boundaries in the Litchfield province are not particularly confused or complicated, nor are the igneous lithologies particularly diverse. There is no evidence for an Archaean age for the part of the Litchfield province north of the Daly River, and it is thus proposed that the term 'Litchfield Complex' should be abandoned. (author)
[en] Metamorphic mineral compositions of a staurolite-bearing greyschist from the middle reaches of the Moeraki River valley in south Westland reveal peak equilibration at c. 558±50 degrees C and c. 6.1±1.2 kbar. Two c. 83 Ma U-Pb monazite age populations from the cores of monazite-apatite-allanite-epidote corona structures in mylonitised schists from near Fox Glacier confirm that Alpine Schist metamorphism occurred during the Late Cretaceous. The published spread in Late Cretaceous metamorphic ages indicates that metamorphism was diachronous or was a protracted event. Further dating is required to pin down the cryptic transition into the Jurassic-Early Cretaceous metamorphosed Otago Schist, but the Alpine Schist must extend at least 11 km east of the Alpine Fault in south Westland and overprint the suture between the Pounamu and Rakaia terranes. The P-T-t results imply that the Late Cretaceous crust represented by portions of the Alpine Schist was probably of similar thickness to that beneath the Southern Alps today, but with dehydration and partial melting occurring near the base. The crust under Westland and Otago may be dry and therefore strong. (author).